I use a Mitel desk phone which connects to my company’s PBX when working from home. I used to hook it up to my iMac and used internet sharing to make it connect. I tweaked the default network of the internal DHCP server used (to avoid a clash with the 2.0 network at my company)  by adding a ‘SharingNetwork’ key to the NAT configuration in /Library/Preferences/SystemConfiguration/com.apple.nat

As of Yosemite this ceased to work and I used an extra wire to my switch to connect the Mitel.

I was certain it should work so I spent some time on google and found the solution http://hints.macworld.com/article.php?story=20090510120814850: